Electron Localization Engineering of Oxygen‐Backfilled Sulfur Vacancies Enables Efficient H 2 O 2 Photosynthesis by Suppressing Decomposition

The rapid decomposition of freshly formed H 2 O 2 or key intermediates (e.g., *OOH, *H 2 O 2 ) on metal sulfide surfaces poses a significant challenge in photocatalytic synthesis, primarily due to their strong adsorption and inherently unbalanced interfacial charge transfer of these catalysts. Here, we engineer electron localization in ZnIn 2 S 4 via synergistic sulfur vacancy creation and oxygen backfilling (O,S v ‐OZIS) to tailor active sites, which effectively suppresses H 2 O 2 decomposition while boosting its photosynthesis by promoting *H 2 O 2 desorption and accelerating charge transfer. In situ XPS and DFT calculations reveal that the catalytic active sites shifts from S atoms in ZIS to Zn atoms adjacent to sulfur vacancies in O,S v ‐OZIS, thereby enhancing *H 2 O 2 desorption and inhibiting O─O bond scission in critical intermediates. Consequently, under light illumination, the H 2 O 2 decomposition rate decreases from 38.17% (pristine ZIS) to 9.42% (O,S v ‐OZIS). Furthermore, scaled‐up (2.0 L batch) and continuous‐flow systems under natural sunlight achieve H 2 O 2 concentrations of 3.3 and 1.42 mM, respectively. The as‐produced H 2 O 2 demonstrates effective performance in pollutant degradation and antibacterial applications. This work highlights electron‐localization engineering via defect tailoring and heteroatom backfilling as a general strategy to balance formation and stability in photocatalytic H 2 O 2 synthesis.